B. Briscoe
BT
T. Moncaster
University of Cambridge
M. Menth
University of Tuebingen
July 2012

Encoding Three Pre-Congestion Notification (PCN) States

in the IP Header Using a Single Diffserv Codepoint (DSCP)

Abstract

The objective of Pre-Congestion Notification (PCN) is to protect the
quality of service (QoS) of inelastic flows within a Diffserv domain.
The overall rate of PCN-traffic is metered on every link in the PCN-
domain, and PCN-packets are appropriately marked when certain
configured rates are exceeded. Egress nodes pass information about
these PCN-marks to Decision Points that then decide whether to admit
or block new flow requests or to terminate some already admitted
flows during serious pre-congestion.

This document specifies how PCN-marks are to be encoded into the IP
header by reusing the Explicit Congestion Notification (ECN)
codepoints within a PCN-domain. The PCN wire protocol for non-IP
protocol headers will need to be defined elsewhere. Nonetheless,
this document clarifies the PCN encoding for MPLS in an informational
appendix. The encoding for IP provides for up to three different PCN
marking states using a single Diffserv codepoint (DSCP): not-marked
(NM), threshold-marked (ThM), and excess-traffic-marked (ETM).
Hence, it is called the 3-in-1 PCN encoding. This document obsoletes
RFC 5696.

Status of This Memo

This is an Internet Standards Track document.

This document is a product of the Internet Engineering Task Force
(IETF). It represents the consensus of the IETF community. It has
received public review and has been approved for publication by the
Internet Engineering Steering Group (IESG). Further information on
Internet Standards is available in Section 2 of RFC 5741.

Information about the current status of this document, any errata,
and how to provide feedback on it may be obtained at
http://www.rfc-editor.org/info/rfc6660.

Copyright Notice

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the Trust Legal Provisions and are provided without warranty as
described in the Simplified BSD License.

1. Introduction

The objective of Pre-Congestion Notification (PCN) [RFC5559] is to
protect the quality of service (QoS) of inelastic flows within a
Diffserv domain in a simple, scalable, and robust fashion. Two
mechanisms are used: admission control, to decide whether to admit or
block a new flow request, and flow termination to terminate some
existing flows during serious pre-congestion. To achieve this, the
overall rate of PCN-traffic is metered on every link in the domain,
and PCN-packets are appropriately marked when certain configured
rates are exceeded. These configured rates are below the rate of the
link, thus providing notification to boundary nodes about overloads
before any real congestion occurs (hence "pre-congestion
notification").

[RFC5670] provides for two metering and marking functions that are
generally configured with different reference rates. Threshold-
marking marks all PCN packets once their traffic rate on a link
exceeds the configured reference rate (PCN-threshold-rate). Excess-
traffic-marking marks only those PCN packets that exceed the
configured reference rate (PCN-excess-rate). The PCN-excess-rate is
typically larger than the PCN-threshold-rate [RFC5559]. Egress nodes
monitor the PCN-marks of received PCN-packets and pass information
about these PCN-marks to Decision Points that then decide whether to
admit new flows or terminate existing flows [RFC6661] [RFC6662].

The encoding defined in [RFC5696] described how two PCN marking
states (not-marked and PCN-marked) could be encoded into the IP
header using a single Diffserv codepoint. It defined '01' as an
experimental codepoint (EXP), along with guidelines for its use. Two
PCN marking states are sufficient for the Single Marking edge
behaviour [RFC6662]. However, PCN-domains utilising the controlled
load edge behaviour [RFC6661] require three PCN marking states. This
document extends the encoding that originally appeared in RFC 5696 by
redefining the experimental codepoint as a third PCN marking state in
the IP header, but still using a single Diffserv codepoint. This
encoding scheme is therefore called the "3-in-1 PCN encoding". It
obsoletes the [RFC5696] encoding, which provides only a subset of the
same capabilities.

The full version of the 3-in-1 encoding requires any tunnel endpoint
within the PCN-domain to support the normal tunnelling rules defined
in [RFC6040]. There is one limited exception to this constraint
where the PCN-domain only uses the excess-traffic-marking behaviour
and where the threshold-marking behaviour is deactivated. This is
discussed in Section 5.2.3.1.

This document only concerns the PCN wire protocol encoding for IP
headers, whether IPv4 or IPv6. It makes no changes or
recommendations concerning algorithms for congestion marking or
congestion response. Other documents will define the PCN wire
protocol for other header types. Appendix C discusses a possible
mapping between IP and MPLS.

1.1. Requirements Language

The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in [RFC2119].

2. Definitions and Abbreviations

2.1. Terminology

The terms PCN-domain, PCN-node, PCN-interior-node, PCN-ingress-node,
PCN-egress-node, PCN-boundary-node, PCN-traffic, PCN-packets, and
PCN-marking are used as defined in [RFC5559]. The following
additional terms are defined in this document:
PCN encoding: mapping of PCN marking states to specific codepoints
in the packet header.
PCN-compatible Diffserv codepoint: a Diffserv codepoint indicating
packets for which the ECN field carries PCN-markings rather than
[RFC3168] markings. Note that an operator configures PCN-nodes to
recognise PCN-compatible DSCPs, whereas the same DSCP has no PCN-
specific meaning to a node outside the PCN-domain.
Threshold-marked codepoint: a codepoint that indicates a packet has
been threshold-marked; that is, a packet that has been marked at a
PCN-interior-node as a result of an indication from the threshold-
metering function [RFC5670]. Abbreviated to ThM codepoint.
Excess-traffic-marked codepoint: a codepoint that indicates packets
that have been marked at a PCN-interior-node as a result of an
indication from the excess-traffic-metering function [RFC5670].
Abbreviated to ETM codepoint.
Not-marked codepoint: a codepoint that indicates PCN-packets that
are not PCN-marked. Abbreviated to NM codepoint.
Not-PCN codepoint: a codepoint that indicates packets that are not
PCN-packets.

Figure 1: 3-in-1 PCN Encoding

A PCN-node will be configured to recognise certain DSCPs as PCN-
compatible. Appendix A discusses the choice of suitable DSCPs. In
Figure 1, 'DSCP n' indicates such a PCN-compatible DSCP. In the PCN-
domain, any packet carrying a PCN-compatible DSCP and with the ECN-
field anything other than 00 (not-PCN) is a PCN-packet as defined in
[RFC5559].

PCN-nodes MUST interpret the ECN field of a PCN-packet using the
3-in-1 PCN encoding, rather than [RFC3168]. This does not change the
behaviour for any packet with a DSCP that is not PCN-compatible, or
for any node outside a PCN-domain. In all such cases, the 3-in-1
encoding is not applicable, and so by default the node will interpret
the ECN field using [RFC3168].

When using the 3-in-1 encoding, the codepoints of the ECN field have
the following meanings:

not-PCN: indicates a non-PCN-packet, i.e., a packet that uses a PCN-
compatible DSCP but is not subject to PCN metering and marking.
NM: not-marked. Indicates a PCN-packet that has not yet been marked
by any PCN marker.
ThM: threshold-marked. Indicates a PCN-packet that has been marked
by a threshold-marker [RFC5670].
ETM: excess-traffic-marked. Indicates a PCN-packet that has been
marked by an excess-traffic-marker [RFC5670].

4. Requirements for and Applicability of 3-in-1 PCN Encoding

4.1. PCN Requirements

In accordance with the PCN architecture [RFC5559], PCN-ingress-nodes
control packets entering a PCN-domain. Packets belonging to PCN-
controlled flows are subject to PCN-metering and PCN-marking, and
PCN-ingress-nodes mark them as not-marked (PCN-colouring). All nodes
in the PCN-domain perform PCN-metering and PCN-mark PCN-packets if
needed. There are two different metering and marking behaviours:
threshold-marking and excess-traffic-marking [RFC5670]. Some edge
behaviours require only a Single Marking behaviour [RFC6662], others
require both [RFC6661]. In the latter case, three PCN marking states
are needed: not-marked (NM) to indicate not-marked packets,
threshold-marked (ThM) to indicate packets marked by the threshold-
marker, and excess-traffic-marked (ETM) to indicate packets marked by
the excess-traffic-marker [RFC5670]. Threshold-marking and excess-
traffic-marking are configured to start marking packets at different
load conditions, so one marking behaviour indicates more severe pre-
congestion than the other. Therefore, a fourth PCN marking state
indicating that a packet is marked by both markers is not needed.
However, a fourth codepoint is required to indicate packets that use
a PCN-compatible DSCP but do not use PCN-marking (the not-PCN
codepoint).

In all current PCN edge behaviours that use two marking behaviours
[RFC5559] [RFC6661], excess-traffic-marking is configured with a
larger reference rate than threshold-marking. We take this as a rule
and define excess-traffic-marked as a more severe PCN-mark than
threshold-marked.

4.2. Requirements Imposed by Tunnelling

[RFC6040] defines rules for the encapsulation and decapsulation of
ECN markings within IP-in-IP tunnels. The publication of RFC 6040
removed the tunnelling constraints that existed when the encoding of
[RFC5696] was written (see Section 3.3.2 of [RFC6627]).

Nonetheless, there is still a problem if there are any legacy (pre-
RFC6040) decapsulating tunnel endpoints within a PCN-domain. If a
PCN-node Threshold-marks the outer header of a tunnelled packet that
has a not-marked codepoint on the inner header, a legacy decapsulator
will forward the packet as not-marked, losing the Threshold-marking.
The rules on applicability in Section 4.3 below are designed to avoid
this problem.

Even if an operator accidentally breaks these applicability rules,
the order of severity of the 3-in-1 codepoints was chosen to protect
other PCN or non-PCN traffic. Although legacy pre-RFC6040 tunnels
did not propagate '01', all tunnels pre-RFC6040 and post-RFC6040 have
always propagated '11' correctly. Therefore, '11' was chosen to
signal the most severe pre-congestion (ETM), so it would act as a
reliable fail-safe even if an overlooked legacy tunnel was
suppressing '01' (ThM) signals.

4.3. Applicable Environments for the 3-in-1 PCN Encoding

The 3-in-1 encoding is applicable in situations where two marking
behaviours are being used in the PCN-domain. The 3-in-1 encoding can
also be used with only one marking behaviour, in which case one of
the codepoints MUST NOT be used anywhere in the PCN-domain (see
Section 5.2.3).

With one exception (see next paragraph), any tunnel endpoints
(IP-in-IP and IPsec) within the PCN-domain MUST comply with the ECN
encapsulation and decapsulation rules set out in [RFC6040] (see
Section 4.2).

Operators may not be able to upgrade every pre-RFC6040 tunnel
endpoint within a PCN-domain. In such circumstances, a limited
version of the 3-in-1 encoding can still be used but only under the
following stringent condition. If any pre-RFC6040 tunnel
decapsulator exists within a PCN-domain, then every PCN-node in the
PCN-domain MUST be configured so that it never sets the ThM
codepoint. PCN-interior-nodes in this case MUST solely use the
Excess-traffic-marking function, as defined in Section 5.2.3.1. In
all other situations where legacy tunnel decapsulators might be
present within the PCN-domain, the 3-in-1 encoding is not applicable.

5. Behaviour of a PCN-node to Comply with the 3-in-1 PCN Encoding

Any tunnel endpoint implementation on a PCN-node MUST comply with
[RFC6040]. Since PCN is a new capability, this is considered a
reasonable requirement.

5.1. PCN-Ingress-Node Behaviour

If packets arrive from another Diffserv domain, any re-mapping of
Diffserv codepoints MUST happen before PCN-ingress processing.

At each logical ingress link into a PCN-domain, each PCN-ingress-node
will apply the four functions described in Section 4.2 of [RFC5559]
to arriving packets. These functions are applied in the following
order: PCN-classify, PCN-police, PCN-colour, PCN-rate-meter. This
section describes these four steps, but only the aspects relevant to
packet encoding:

1. PCN-classification: The PCN-ingress-node determines whether each
packet matches the filter spec of an admitted flow. Packets that
match are defined as PCN-packets.
1b. Extra step if ECN and PCN coexist: If a packet classified as a
PCN-packet arrives with the ECN field already set to a value other
than Not-ECT (i.e., it is from an ECN-capable transport), then to
comply with BCP 124 [RFC4774] it MUST pass through one of the
following preparatory steps before the PCN-policing and PCN-
colouring steps. The choice between these four actions depends on
local policy:

or within a lower-layer protocol capable of being PCN-
marked, such as MPLS (see Appendix C).

Encapsulation using either of these methods is the RECOMMENDED
policy for ECN-capable PCN-packets, and implementations SHOULD
use IP-in-IP tunnelling as the default.

If encapsulation is used, it MUST precede PCN-policing and PCN-
colouring so that the encapsulator and decapsulator are
logically outside the PCN-domain (see Appendix B and
specifically Figure 2).

If MPLS encapsulation is used, note that penultimate hop
popping [RFC3031] is incompatible with PCN, unless the
penultimate hop applies the PCN-egress-node behaviour before it
pops the PCN-capable MPLS label.

If some form of encapsulation is not possible, the PCN-ingress-
node can allow through ECN-capable packets without
encapsulation, but it MUST drop CE-marked packets at this
stage. Failure to drop CE-marked packets would risk congestion
collapse, because without encapsulation there is no mechanism
to propagate the CE markings across the PCN-domain (see
Appendix B).

This policy is NOT RECOMMENDED because there is no tunnel to
protect the e2e ECN capability, which is otherwise disabled
when the PCN-egress-node zeroes the ECN field.

Drop the packet.

This policy is also NOT RECOMMENDED, because it precludes the
possibility that e2e ECN can coexist with PCN as a means of
controlling congestion.

Any other action that complies with [RFC4774] (see Appendix B
for an example).

Appendix B provides more information about the coexistence of PCN
and ECN.

2. PCN-policing: The PCN-policing function only allows appropriate
packets into the PCN behaviour aggregate. Per-flow policing
actions may be required to block rejected flows and to rate-police
accepted flows, but these are specified in the relevant edge-
behaviour document, e.g., [RFC6662] or [RFC6661].

Here, we only specify packet-level PCN-policing, which prevents
packets that are not PCN-packets from being forwarded into the
PCN-domain if PCN-interior-nodes would otherwise mistake them for
PCN-packets. A non-PCN-packet will be confused with a PCN-packet
if on arrival it meets all three of the following conditions:

a) it is not classified as a PCN-packet;

b) it already carries a PCN-compatible DSCP; and

c) its ECN field carries a codepoint other than Not-ECT.

The PCN-ingress-node MUST police packets that meet all three
conditions (a-c) by subjecting them to one of the following
treatments:

re-mark the DSCP to a DSCP that is not PCN-compatible;

tunnel the packet to the PCN-egress-node with a DSCP in the
outer header that is not PCN-compatible; or

drop the packet (NOT RECOMMENDED -- see below).

The choice between these actions depends on local policy. In the
absence of any operator-specific configuration for this case, an
implementation SHOULD re-mark the DSCP to zero (000000) by
default.

Whichever policing action is chosen, the PCN-ingress-node SHOULD
log the event and MAY also raise an alarm. Alarms SHOULD be rate-
limited so that the anomalous packets will not amplify into a
flood of alarm messages.

Rationale: Traffic that meets all three of the above conditions
(a-c) is not PCN-traffic; therefore, ideally a PCN-ingress ought
not to interfere with it, but it has to do something to avoid
ambiguous packet markings. Clearing the ECN field is not an
appropriate policing action, because a network node ought not to
interfere with an e2e signal. Even if such packets seem like an
attack, drop would be overkill, because such an attack can be
neutralised by just re-marking the DSCP. And DSCP re-marking in
the network is legitimate, because the DSCP is not considered an
e2e signal.

3. PCN-colouring: If a packet has been classified as a PCN-packet,
once it has been policed, the PCN-ingress-node:

MUST set a PCN-compatible Diffserv codepoint on all PCN-
packets. To conserve DSCPs, DSCPs SHOULD be chosen that are
already defined for use with admission-controlled traffic.
Appendix A gives guidance to implementors on suitable DSCPs.

MUST set the PCN codepoint of all PCN-packets to not-marked
(NM).

4. PCN rate-metering: This fourth step may be necessary depending on
the edge behaviour in force. It is listed for completeness, but
it is not relevant to this encoding document.

5.2. PCN-Interior-Node Behaviour

5.2.1. Behaviour Common to All PCN-Interior-Nodes

Interior nodes MUST NOT change not-PCN to any other codepoint.

Interior nodes MUST NOT change NM to not-PCN.

Interior nodes MUST NOT change ThM to NM or not-PCN.

Interior nodes MUST NOT change ETM to any other codepoint.

5.2.2. Behaviour of PCN-Interior-Nodes Using Two PCN-Markings

If the threshold-meter function indicates a need to mark a packet,
the PCN-interior-node MUST change NM to ThM.

If the excess-traffic-meter function indicates a need to mark a
packet:

the PCN-interior-node MUST change NM to ETM;

the PCN-interior-node MUST change ThM to ETM.

If both the threshold meter and the excess-traffic meter indicate the
need to mark a packet, the Excess-traffic-marking rules MUST take
precedence.

5.2.3. Behaviour of PCN-Interior-Nodes Using One PCN-Marking

Some PCN edge behaviours require only one PCN-marking within the PCN-
domain. The Single Marking edge behaviour [RFC6662] requires PCN-
interior-nodes to mark packets using the excess-traffic-meter
function [RFC5670]. It is possible that future schemes may require
only the threshold-meter function. Appendix D explains the rationale
for the behaviours defined in this section.

5.2.3.1. Marking Using Only the Excess-Traffic-Meter Function

The threshold-traffic-meter function SHOULD be disabled and MUST NOT
trigger any packet marking.

The PCN-interior-node SHOULD raise a management alarm if it receives
a ThM packet, but the frequency of such alarms SHOULD be limited.

If the excess-traffic-meter function indicates a need to mark the
packet:

the PCN-interior-node MUST change NM to ETM;

the PCN-interior-node MUST change ThM to ETM. It SHOULD also
raise an alarm as above.

5.2.3.2. Marking Using Only the Threshold-Meter Function

The excess-traffic-meter function SHOULD be disabled and MUST NOT
trigger any packet marking.

The PCN-interior-node SHOULD raise a management alarm if it receives
an ETM packet, but the frequency of such alarms SHOULD be limited.

If the threshold-meter function indicates a need to mark the packet:

the PCN-interior-node MUST change NM to ThM;

the PCN-interior-node MUST NOT change ETM to any other codepoint.
It SHOULD raise an alarm as above if it encounters an ETM packet.

5.3. PCN-Egress-Node Behaviour

A PCN-egress-node SHOULD set the not-PCN ('00') codepoint on all
packets it forwards out of the PCN-domain.

The only exception to this is if the PCN-egress-node is certain that
revealing other codepoints outside the PCN-domain won't contravene
the guidance given in [RFC4774]. For instance, if the PCN-ingress-
node has explicitly informed the PCN-egress-node that this flow is
ECN-capable, then it might be safe to expose other ECN codepoints.
Appendix B gives details of how such schemes might work, but such
schemes are currently only tentative ideas.

If the PCN-domain is configured to use only Excess-traffic-marking,
the PCN-egress-node MUST treat ThM as ETM; if only threshold-marking
is used, it SHOULD treat ETM as ThM. However, it SHOULD raise a
management alarm in either case since this means there is some
misconfiguration in the PCN-domain.

6. Backward Compatibility

6.1. Backward Compatibility with ECN

BCP 124 [RFC4774] gives guidelines for specifying alternative
semantics for the ECN field. It sets out a number of factors to be
taken into consideration. It also suggests various techniques to
allow the coexistence of default ECN and alternative ECN semantics.
The encoding specified in this document uses one of those techniques;
it defines PCN-compatible Diffserv codepoints as no longer supporting
the default ECN semantics within a PCN-domain. As such, this
document is compatible with BCP 124.

There is not enough space in one IP header for the 3-in-1 encoding to
support both ECN marking end-to-end and PCN-marking within a PCN-
domain. The non-normative Appendix B discusses possible ways to do
this, e.g., by carrying e2e ECN across a PCN-domain within the inner
header of an IP-in-IP tunnel. The normative text in Section 5.1
requires one of these methods to be configured at the PCN-ingress-
node and recommends that implementations offer tunnelling as the
default.

In any PCN deployment, traffic can only enter the PCN-domain through
PCN-ingress-nodes and leave through PCN-egress-nodes. PCN-ingress-
nodes ensure that any packets entering the PCN-domain have the ECN
field in their outermost IP header set to the appropriate codepoint.

PCN-egress-nodes then guarantee that the ECN field of any packet
leaving the PCN-domain has appropriate ECN semantics. This prevents
unintended leakage of ECN marks into or out of the PCN-domain, and
thus reduces backward-compatibility issues.

Section 5.1 of the PCN architecture [RFC5559] gives general guidance
on fault detection and diagnosis, including management analysis of
PCN markings arriving at PCN-egress-nodes to detect early signs of
potential faults. Because the PCN encoding has gone through an
obsoleted earlier stage [RFC5696], misconfiguration mistakes may be
more likely. Therefore, extra monitoring, such as in the following
example, may be necessary to detect and diagnose potential problems:

Informational example: In a controlled-load edge-behaviour
scenario it could be worth the PCN-egress-node detecting the onset
of excess-traffic marking without any prior threshold-marking.
This might indicate that an interior node has been wrongly
configured to mark only ETM (which would have been correct for the
single-marking edge behaviour).

A PCN-node implemented to use the obsoleted encoding in RFC 5696
could conceivably have been configured so that the Threshold-meter
function marked what is now defined as the ETM codepoint in the
3-in-1 encoding. However, there is no known deployment of this
rather unlikely variant of RFC 5696 and no reason to believe that
such an implementation would ever have been built. Therefore, it
seems safe to ignore this issue.

7. Security Considerations

PCN-marking only carries a meaning within the confines of a PCN-
domain. This encoding document is intended to stand independently of
the architecture used to determine how specific packets are
authorised to be PCN-marked, which will be described in separate
documents on PCN-boundary-node behaviour.

This document assumes the PCN-domain to be entirely under the control
of a single operator, or a set of operators who trust each other.
However, future extensions to PCN might include inter-domain versions
where trust cannot be assumed between domains. If such schemes are
proposed, they must ensure that they can operate securely despite the
lack of trust. However, such considerations are beyond the scope of
this document.

One potential security concern is the injection of spurious PCN-marks
into the PCN-domain. However, these can only enter the domain if a
PCN-ingress-node is misconfigured. The precise impact of any such
misconfiguration will depend on which of the proposed PCN-boundary-
node behaviours is used; however, in general, spurious marks will
lead to admitting fewer flows into the domain or potentially
terminating too many flows. In either case, good management should
be able to quickly spot the problem since the overall utilisation of
the domain will rapidly fall.

8. Conclusions

The 3-in-1 PCN encoding uses a PCN-compatible DSCP and the ECN field
to encode PCN-marks. One codepoint allows non-PCN traffic to be
carried with the same PCN-compatible DSCP and three other codepoints
support three PCN marking states with different levels of severity.
In general, the use of this PCN encoding scheme presupposes that any
tunnel endpoints within the PCN-domain comply with [RFC6040].

9. Acknowledgements

Many thanks to Philip Eardley for providing extensive feedback,
criticism and advice. Thanks also to Teco Boot, Kwok Ho Chan,
Ruediger Geib, Georgios Karagiannis, James Polk, Tom Taylor, Adrian
Farrel, and everyone else who has commented on the document.

Appendix A. Choice of Suitable DSCPs

This appendix is informative not normative.

A single DSCP has not been defined for use with PCN for several
reasons. Firstly, the PCN mechanism is applicable to a variety of
different traffic classes. Secondly, Standards Track DSCPs are in
increasingly short supply. Thirdly, PCN is not a scheduling
behaviour -- rather, it should be seen as being a marking behaviour
similar to ECN but intended for inelastic traffic. The choice of
which DSCP is most suitable for a given PCN-domain is dependent on
the nature of the traffic entering that domain and the link rates of
all the links making up that domain. In PCN-domains with sufficient
aggregation, the appropriate DSCPs would currently be those for the
Real-Time Treatment Aggregate [RFC5127]. It is suggested that
admission control could be used for the following service classes
(defined in [RFC4594] unless otherwise stated):

CS5 is excluded from this list since PCN is not expected to be
applied to signalling traffic.

PCN-marking is intended to provide a scalable admission-control
mechanism for traffic with a high degree of statistical multiplexing.
PCN-marking would therefore be appropriate to apply to traffic in the
above classes, but only within a PCN-domain containing sufficiently
aggregated traffic. In such cases, the above service classes may
well all be subject to a single forwarding treatment (treatment
aggregate [RFC5127]). However, this does not imply all such IP
traffic would necessarily be identified by one DSCP -- each service
class might keep a distinct DSCP within the highly aggregated region
[RFC5127].

Guidelines for conserving DSCPs by allowing non-admission-controlled-
traffic to compete with PCN-traffic are given in Appendix B.1 of
[RFC5670].

Additional service classes may be defined for which admission control
is appropriate, whether through some future standards action or
through local use by certain operators, e.g., the Multimedia
Streaming service class (AF3). This document does not preclude the
use of PCN in more cases than those listed above.

Note: The above discussion is informative not normative, as operators
are ultimately free to decide whether to use admission control for
certain service classes and whether to use PCN as their mechanism of
choice.

Appendix B. Coexistence of ECN and PCN

This appendix is informative not normative. It collects together
material relevant to coexistence of ECN and PCN, including that
spread throughout the body of this specification. If this results in
any conflict or ambiguity, the normative text in the body of the
specification takes precedence.

ECN [RFC3168] is an e2e congestion notification mechanism. As such
it is possible that some traffic entering the PCN-domain may also be
ECN-capable. The PCN encoding described in this document reuses the
bits of the ECN field in the IP header. Consequently, this disables
ECN within the PCN-domain.

For the purposes of this appendix, we define two forms of traffic
that might arrive at a PCN-ingress-node. These are admission-
controlled traffic (PCN-traffic) and non-admission-controlled traffic
(non-PCN-traffic).

Flow signalling identifies admission-controlled traffic, by
associating a filter spec with the need for admission control (e.g.,
through RSVP or some equivalent message, such as from a SIP server to
the ingress or from a logically centralised network control system).
The PCN-ingress-node re-marks admission-controlled traffic matching
that filter spec to a PCN-compatible DSCP. Note that the term "flow"
need not imply just one microflow, but instead could match an
aggregate and/or could depend on the incoming DSCP (see Appendix A).

All other traffic can be thought of as non-admission-controlled (and
therefore outside the scope of PCN). However, such traffic may still
need to share the same DSCP as the admission-controlled traffic.
This may be due to policy (for instance, if it is high-priority voice
traffic), or may be because there is a shortage of local DSCPs.

Unless specified otherwise, for any of the cases in the list below,
an IP-in-IP tunnel that complies with [RFC6040] can be used to
preserve ECN markings across the PCN-domain. The tunnelling action
should be applied wholly outside the PCN-domain as illustrated in
Figure 2. Then, by the rules of RFC 6040, the tunnel egress
propagates the ECN field from the inner header, because the PCN-
egress will have zeroed the outer ECN field.

Figure 2: Separation of Tunnelling and PCN Actions

There are three cases for how e2e ECN traffic may wish to be treated
while crossing a PCN-domain:

a) Traffic that does not require PCN admission control:

For example, traffic that does not match flow signalling being
used for admission control. In this case, the e2e ECN traffic is
not treated as PCN-traffic. There are two possible scenarios:

Arriving traffic does not carry a PCN-compatible DSCP: no
action required.

Arriving traffic carries a DSCP that clashes with a PCN-
compatible DSCP. There are two options:

The ingress maps the DSCP to a local DSCP with the same
scheduling PHB as the original DSCP, and the egress re-maps
it to the original PCN-compatible DSCP.

The ingress tunnels the traffic, setting the DSCP in the
outer header to a local DSCP with the same scheduling PHB
as the original DSCP.

The ingress tunnels the traffic, using the original DSCP in
the outer header but setting not-PCN in the outer header;
note that this turns off ECN for this traffic within the
PCN-domain.

The first or second options are recommended unless the operator
is short of local DSCPs.

b) Traffic that requires admission-control:

In this case, the e2e ECN traffic is treated as PCN-traffic across
the PCN-domain. There are two options.

The PCN-ingress-node places this traffic in a tunnel with a
PCN-compatible DSCP in the outer header. The PCN-egress zeroes
the ECN-field before decapsulation.

* The PCN-ingress-node drops CE-marked packets and otherwise sets
the ECN-field to NM and sets the DSCP to a PCN-compatible DSCP.
The PCN-egress zeroes the ECN field of all PCN packets; note
that this turns off e2e ECN.

The second option is emphatically not recommended, unless perhaps
as a last resort if tunnelling is not possible for some
insurmountable reason.

c) Traffic that requires PCN admission control where the endpoints

ask to see PCN marks:
Note that this scheme is currently only a tentative idea.

For real-time data generated by an adaptive codec, schemes have
been suggested where PCN marks may be leaked out of the PCN-domain
so that end hosts can drop to a lower data-rate, thus deferring
the need for admission control. Currently, such schemes require
further study and the following is for guidance only.

The PCN-ingress-node needs to tunnel the traffic as in Figure 2,
taking care to comply with [RFC6040]. In this case, the PCN-
egress does not zero the ECN field (contrary to the recommendation
in Section 5.3), so that the [RFC6040] tunnel egress will preserve
any PCN-marking. Note that a PCN-interior-node may change the
ECN-field from '10' to '01' (NM to ThM), which would be
interpreted by the e2e ECN as a change from ECT(0) into ECT(1).
This would not be compatible with the (currently experimental) ECN
nonce [RFC3540].

Appendix C. Example Mapping between Encoding of PCN-Marks in IP and in

MPLS Shim Headers

This appendix is informative not normative.

The 6 bits of the DS field in the IP header provide for 64
codepoints. When encapsulating IP traffic in MPLS, it is useful to
make the DS field information accessible in the MPLS header.
However, the MPLS shim header has only a 3-bit traffic class (TC)
field [RFC5462] providing for 8 codepoints. The operator has the
freedom to define a site-local mapping of the 64 codepoints of the DS
field onto the 8 codepoints in the TC field.

[RFC5129] describes how ECN markings in the IP header can also be
mapped to codepoints in the MPLS TC field. Appendix A of [RFC5129]
gives an informative description of how to support PCN in MPLS by
extending the way MPLS supports ECN. [RFC5129] was written while PCN
specifications were in early draft stages. The following provides a
clearer example of a mapping between PCN in IP and in MPLS using the
PCN terminology and concepts that have since been specified.

To support PCN in a MPLS domain, a PCN-compatible DSCP ('DSCP n')
needs codepoints to be provided in the TC field for all the PCN-marks
used. That means, when, for instance, only excess-traffic-marking is
used for PCN purposes, the operator needs to define a site-local
mapping to two codepoints in the MPLS TC field for IP headers with:

o DSCP n and NM
o DSCP n and ETM

If both excess-traffic-marking and threshold-marking are used, the
operator needs to define a site-local mapping to codepoints in the
MPLS TC field for IP headers with all three of the 3-in-1 codepoints:

o DSCP n and NM
o DSCP n and ThM
o DSCP n and ETM

In either case, if the operator wishes to support the same Diffserv
PHB but without PCN marking, it will also be necessary to define a
site-local mapping to an MPLS TC codepoint for IP headers marked
with:

DSCP n and not-PCN

The above sets of codepoints are required for every PCN-compatible
DSCP. Clearly, given so few TC codepoints are available, it may be
necessary to compromise by merging together some capabilities.
Guidelines for conserving TC codepoints by allowing non-admission-
controlled-traffic to compete with PCN-traffic are given in Appendix
B.1 of [RFC5670].

Appendix D. Rationale for Difference between the Schemes Using One

PCN-Marking

Readers may notice a difference between the two behaviours in
Sections 5.2.3.1 and 5.2.3.2. With only Excess-traffic-marking
enabled, an unexpected ThM packet can be re-marked to ETM. However,
with only Threshold-marking, an unexpected ETM packet cannot be re-
marked to ThM.

This apparent inconsistency is deliberate. The behaviour with only
Threshold-marking keeps to the rule of Section 5.2.1 that ETM is more
severe and must never be changed to ThM even though ETM is not a
valid marking in this case. Otherwise, implementations would have to
allow operators to configure an exception to this rule, which would
not be safe practice and would require different code in the data
plane compared to the common behaviour.